This invention relates to medical devices and, more particularly, but not exclusively, to devices for performing minimally-invasive surgical procedures.
When performing medical operations, it is of paramount importance that the surgery itself causes as little trauma to the patient as possible. For this reason, medical science is continually developing new and improved methods for treating patients and reducing the risk of severe consequences arising from the procedure itself. In recent years, techniques such as keyhole surgery and endoluminal or transluminal treatments, which avoid the need for traumatic open surgery, have become common practice in many parts of the world. Benefits to the patient include reduced external scarring, minimal trauma during surgery, reduced risk of infection and shorter recovery periods. Correspondingly, there has been a demand for new and improved surgical equipment, capable of performing minimally invasively the functions required to successfully treat the numerous and varying medical conditions. As a result, there is ever-increasing pressure for individual instruments to combine an increasing range of functions, to be of smaller dimensions and to maintain or improve the accuracy and manipulability of the devices.
In the specific field of transluminal or endoluminal surgery, one drive is towards reducing the diameter of devices to be inserted into and guided along a body lumen, in order to allow surgical procedures to be performed in narrow conduits such as blood vessels which are inaccessible to larger devices. At the same time, it is necessary to ensure that the devices can perform to a high level of accuracy and can be easily directed and controlled by the surgeon, particularly when they have to be guided through the tortuous passageways such as are defined by inter-connecting blood vessels within the human body.
Common procedures include the treatment of coronary diseases and deficiencies by the endoluminal implantation and deployment of stents or replacement valve structures at locations where the natural blood vessels have become defective, blocked or damaged. In such a procedure, a medical device is loaded onto or into a delivery catheter in a compressed or reduced-diameter configuration. The catheter is then inserted through an incision into a blood vessel, typically the femoral artery, and guided through the passageway of inter-connecting blood vessels to the site requiring treatment. The medical device then expands or is expanded within the passageway at the treatment site, where it remains. The delivery catheter is then retracted through the passageway and removed through the same incision.
EP-A-0 836 447 discloses a stent delivery system comprising an inner core, having a proximal end and a distal end, made from a wire coil; a stent concentrically arranged around the inner coil near the distal end; a sheath concentrically arranged around the inner coil extending from the proximal end to a distal end proximal of the stent; an outer sheath covering the stent; and means for retracting the outer sheath. When the delivery system has been inserted and guided to the correct location, the outer sheath is retracted, releasing the stent contained therein. During insertion of the catheter, the wire coil is flexible, allowing it to advance through the tortuous passageway defined by the inter-connecting blood vessels. During retraction of the outer sheath, the wire coil provides sufficient rigidity and resistance to axial compression to allow retraction of the outer sheath.
EP-A-1 181 906 discloses a similar stent delivery catheter, including a wire coil and further including a covering that fits over the coil to help resist buckling in bending and compression.
There is associated with endoluminal surgical procedures the need to visualize the position of the catheter as it is advanced through the bodily lumen, and to ensure that the device is properly located prior to, during and after deployment. In order to visualize the position of the advanced distal end of the catheter, pulses of visualizing fluid are injected into the bodily lumen so that the catheter distal end can be seen using visualisation means, such as radioscopy or fluoroscopy. In order to transport the visualizing fluid, prior art devices either provide a separate lumen within the catheter, for transporting visualizing fluid from the proximal end to the distal end of the catheter, or visualizing fluid may be injected through a guide catheter, within which the delivery catheter is advanced. In the latter system, the catheter can be provided with a shaft that is narrow except at the distal end, where the medical device is held, so that a sufficient volume of visualizing fluid can flow within the guide catheter lumen around the shaft and is not restricted until it reaches the distal end. A continuing preoccupation for catheter designs is how to provide sufficient quantity of visualizing fluid from a proximal end to a distal end of the delivery catheter without requiring an increased-diameter catheter and without restricting the flow past the delivery catheter.
With the prior art devices, there is often a trade-off between the flexibility of the delivery catheter and the resistance of the delivery catheter to compressive forces during retraction of the sheath containing the medical device. One method of mitigating this difficulty has been to reduce the length of the sheath relative to that of the medical device, and thereby reduce the friction force resisting retraction of the sheath during deployment of the medical device. A pull wire within the catheter can be used for controlled retraction of the sheath once the catheter is correctly positioned. In some prior art devices, the outer sheath is braided or has wire reinforcement to prevent the sheath from lengthwise stretching, so that the sheath will retract when the actuating means is operated and not just become longitudinally stretched. This results in the sheath having increased thickness. Therefore, another problem is to provide a system with reduced overall diameter that is flexible and controllable, without losing compression-resistance in the inner coil, and without the sheath stretching during retraction and incorrectly deploying or failing to deploy the medical device.